Zaxxon was ported to PC (early 80's I believe). It was an isometric shooter. Not as frenetic as your typical shmup, but it had a unique perspective and ability to fly up and down, in addition to left and right.

Overkill but that's 1992 that's a year way too new but it's way better than Xenon 2 IMO

Voodoo2s aren't 100mhz stockGeforce256 isn't released as a beta on New Years '99 under the Quadro brandDOS gaming isn't a bilinear 320x200 16:10DOS PCs aren't better than the MacintoshDOSBox is not for running Windows 9xSGL != Glide

They did make version 1.5 freeware.. but then they started selling it again later and then the freeware version just about disappeared off the internet. But.. if you actually want/need ANY of the versions of Galactix, I do have them. Took me absolutely forever to track down all the versions.

They did change the graphics quite a bit from version 1.0 to version 1.5.. and they also added code to fix the running too fast issue on faster computers in some of the later versions.

Overkill is short, but it's probably my favorite shmup on DOS. The EGA pixel art is awesome, and the soundtrack kicks ass (especially with an MT-32.) Highly recommended.

Star Goose (or Stargoose Warrior) is also worth a look, it's fun & had some interesting gameplay ideas. Xerix and Xerix II are pretty amateurish but do the job, and should work on your target system. Kiloblaster will probably run fine too, but it's awful. Unpolished mess.

I do remember Tubular Worlds. I definitely played this in the 90s. But not excessively, I think and also on my later machine, which was a 486SX-33. I will have a look anyway. I think Overkill looks like the best of the bunch, indeed. Star Goose does not ring a bell, but looks worthwile.

For a 286 you'll want EGA anyway.... even many 386s struggled with VGA shmups (some of my friends couldn't run Raptor on release)

Voodoo2s aren't 100mhz stockGeforce256 isn't released as a beta on New Years '99 under the Quadro brandDOS gaming isn't a bilinear 320x200 16:10DOS PCs aren't better than the MacintoshDOSBox is not for running Windows 9xSGL != Glide

Yeah, raptor and Tyrian are another league... it's interesting how few games made use of VGA hardware scrolling to alleviate a bit the CPU load. Pinball Fantasies/Dreams come to mind, but they were 386+ as well.